How Does Stretch Wrapping Work? - Lantech

To understand how stretch wrapping works you first have to understand why we stretch wrap loads in the first place.

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Why do we stretch wrap?

There are many reasons why we stretch wrap. One main reason is unitizing a load for shipment. Building one large package from many small packages makes it easier to transport, ship and protect individual products from damage. Preventing damage is key to getting loads from their point of origin to their destination in “as made” condition. Stretch wrapping allows us to achieve both goals (unitization for easy transportation and delivering goods without damage) in the most cost effective way.

Stretch wrapping loads keeps them from moving and shifting during transportation which is a major source of damage. There are certainly other ways for loads to be damaged during shipment, but stretch wrapping provides a measure of protection that increases the odds of “as made” condition arrival. In turn, that means lower returns, administration duties, credit due to receiver and rework costs – not to mention happier customers and colleagues.

In the ’s, stretch wrapping machines established themselves as the preferred packaging method for unitizing goods for shipment. Meaning that any company that deals with production, reselling, or warehousing of products probably has some familiarity with stretch wrapping.

To make the best decisions about stretch wrapping machines and film, however, it is useful to understand how the mechanics of stretch wrapping work.

Stretch wrapping begins with pallet loading

Before the stretch wrapping process can begin a load must first be assembled on a pallet. Once the load is on the pallet, the whole ensemble (pallet and load) is stretch wrapped together.

Assembling the load on the pallet is an important step. Load placement on the pallet is key. The load should not overhang the sides of the pallet or be more than four inches inboard. The pallet itself also plays a role and should be in good condition.

It is important when stretch wrapping not to skip steps that secure the load the pallet. One options is to wrap all the way down to the bottom of the pallet. However, this runs the risk of film punctures from forklifts or pallet jacks when picking up the load. Such large punctures to the film can propagate up the load, causing a loss of containment force and increasing the risk for load failure.

Pallet Grip® is a great solution to lock the load to the pallet and avoid film punctures. The bottom few inches of the film web are automatically rolled into a tight cable at the end of the wrap cycle.  The cable is driven down just below the top deck boards of the pallet. This strong cable locks the load to the pallet and leaves enough room to clear forks and avoid the puncture problem.

3 ways to apply stretch wrap

There are three ways to apply stretch wrap to a pallet of products: manual, semi-automatic stretch wrapper, or high speed automatic stretch wrapper.

Hand wrapping, as the name implies, relies on people to manually stretch wrap a load. A worker secures the film to the pallet and then walks the film around the load.

Pulling a heavy roll of film around a pallet is wrapping is physically demanding – especially if done right. Imagine the awful dizzy feeling in a hot warehouse. Hand wrapping generally has the highest material cost because there is no consistency of film usage and no substantial pre-stretch. It is also very difficult to wrap a safe-to-ship load consistently.

Semi-automatic stretch wrappers come in two different machine styles – turntable and straddle. The load is place on or in the stretch wrappers wrap zone and stretch film is applied. Turntable models will rotate the load on a turntable while straddle style models have a wrap arm that will move around the load and apply film.

High speed automated stretch wrappers are similar to semi-automatic styles in that a machine performs the actual stretch wrapping of a load. However, automatic stretch wrappers typically include infeed and exit conveyors to automatically cycle loads through the machine.

There are a number of features that can be added to semi-automatic machines that offer some increased automation.  For instance, automatically cutting and clamping the film and a remote start lanyard allow fork truck operators to remain safely buckled in their seats on the trucks even while wrapping loads on a semi automatic machine.

For more help choosing the right level of automation and machine style check out our Stretch Wrapping Fundamentals pages. That’s actually the critical factor to consider – how many loads/day may not capture the surges which often occur late in the day as shipments are picked up. So loads/hour is an important factor.

It’s all about containment force

No matter which type of stretch wrapper you use, containment force is the most important things to get right. Containment force can mean the difference between a load arriving at its destination intact and arriving damaged and unsaleable. So what is containment force?

Containment force is what holds the load together. Officially, it’s the wrap force times the number of film layers. Wrap force is simply a measure of how tightly each layer of film is wrapped around the load. Film layers are the number of wraps applied to a load at a specific point.

There is no magic formula to determine the right amount of containment force. It is done through trial and error.  Wrap, test, change and repeat until you can safely say loads are arriving without damage. Using our over 45 years of experience we developed some Containment Force Guidelines. These are not hard and fast rules, but this chart will give you a good place to start.

Once you’ve found the right amount of containment force works and the stretch wrapper’s settings have been set, it is always a good idea to check periodically to ensure every load is being wrapped correctly. We recommend measuring the containment force once a shift for most operations. It is possible the machine was tampered with accidentally or to resolve other issues the machine may have been experiencing, for example repetitive film breaks after a film roll change.

This post was published on June 27, and updated on December 4, .

Asking how does stretch wrapping work? You first have to understand why we stretch wrap loads in the first place.

Contact us to discuss your requirements of Stretch Film. Our experienced sales team can help you identify the options that best suit your needs.

*Updated 12-8-17*

What do you know?

Do you know the industry terms or engineered properties when it comes to stretch wrap (film)?

Do you know how to achieve the best load containment ratio or how to what the difference is between cast and blown wrap?

Do you know what questions to ask your vendor when you're buying stretch wrap? Like about how you might be able to actually downguage your wrap, meaning saving money while increasing your wrapped pallet's integrity?

Stretch wrap is a highly engineered product that is meant to provide unitization, protection, and containment properties to your pallet loads. But without the right knowledge, you may be throwing your money away, not getting what you need for prices that are way to high.

The basics of stretch wrap

When it comes to stretch wrap, it's critical to know some basic terminology.  Here are some of the most common terms used, as well as a downloadable sheet with additional terms available here.

Banding: The act of applying multiple wraps of stretch film to a certain area of a load to reinforce and/or unitize a number of layers or products. Banding is used when loads need to be completely secure. Banding packaging makes it much easier to ensure the product will reach its destination in good condition.

Blown stretch wrap: A film extruded by the blown balloon type inflation system. Beads of resin are fed through a heated machine that has a circular die. The heated resin is forced through the die and then blown out vertically into a bubble. As this formed bubble finishes the process of being transformed into rolls of stretch film, it is cooled by the surrounding air. This type of blown stretch film generally costs more to make because the output per hour is less than with cast films.Typically blown film is a tougher but noisier film than cast films.

Cast stretch wrap: Molten plastic extruded over full width of a die, then cooled and crystallized over a drum. Cast film typically is quieter, more transparent, and glossier than blown film because of this process. Stretch film extruded by the cast method typically have greater tear resistance, greater aesthetic values and are quiet to unwind, which is not typically found in film manufactured by the blown method.

Cling (single side - two sided - differential): A bonding agent added to stretch film to increase the stickiness quality of the film. This is desirable to allow the layers of stretch film applied to bond to the previous layers effectively creating a single wall of stretch film. Depending on the desired effect the bonding agent is applied to one or both sides. It is most commonly applied using co-extrusion where the bonding agent is a layer that is co-extruded during the manufacturing process.

Film force (film tension): The retaining force applied by the stretch film on the product being wrapped. This force is typically measured in pounds. Film force is created two ways. The second method is by delaying the film feeding out of the carriage. During wrapping the film is fed out at a constant tension. By delaying the response (speed) of the carriage film feed, you electronically increase the film tension. A film force dial is typically located on the control panel or carriage.

Film force release: Typically used on automatic stretch wrapping equipment whereby the film force feature is disabled for a time at the beginning and end of the cycle. This feature prevents unwanted tension at the two points during the cycle where added tension would create problems. The stretch film pulling out of the clamp at the beginning of the cycle and the detaching of the trailing tail at the end of the cycle are typically remedied by film force release.

Film memory:  The key reason for prestretching stretch film. Prestretching stretch film creates a memory in the film, which causes a continuous elastic effect as the film tries to return to its unstretched self. This ensures that the load integrity is maintained even as the load shifts or settles during transit. It is this film memory that differentiates prestretched film from non-prestretched film or other means of unitizing. For instance, settling that occurs during shipment can loosen other methods of unitizing (strapping) where prestretched film memory takes up the slack and continues to secure the load.

Film recovery: Describes the extent in which a material returns to its original shape and size after having been deformed or subject to stretching. This assures that the film will maintain a tight load during transportation.  

Force to load: Varies by number of wraps, gauge being used, force to load settings on machine, pre-stretch level of machine. The most typical force to load seen is 15-26 lbs in the field.

Gauge: A unit of measurement that is used to measure stretch film thickness or caliper. An example value would read as: 40 gauge, 60 gauge or 100 gauge. Also note: Often used as a synonym for film thickness.

Load types (A-load, B-Load, C-Load): Stretch film and machine manufacturers have divided the types of loads wrapped into three load types based on the degree of difficulty to wrap the load or product.

• A-load:  This load is normally cubed with no protrusions.This type of load has clean edges and are the easiest to wrap.
• B-load: This load type may have a few protrusions creating irregular sides. These pallets will need a better performance film that offers more puncture resistance.
• C-load: The most difficult type of load. These pallets are very irregular with many corners and sharp edges that are particularly difficult to wrap. This type of load requires a high performance film.

Neckdown: As film is stretched there is a tendency for the film to narrow (similar to bubble gum). This narrowing of the film is called neckdown. Neckdown reduces the coverage a revolution of stretch film provides thus potentially increasing the number of revolutions required to wrap a pallet or load. The larger the distance between the two rollers stretching the film (Primary & Secondary prestretch rollers) the larger the neckdown.

Overlap: As stretch film is applied to a load the stretch film is typically applied so that the next layer of stretch film is applied over the previous layer. This overlapping of film layers increases load retention. By slowing the vertical movement of the stretch film carriage the larger the overlap created.

Overwrap: The amount of stretch film applied over the top of the load. As the stretch film reaches the top of the load, the vertical movement of the carriage can continue so that the film angles over the top of the load. The stretch film on top creates a downward force on the pallet load. This is also used when a plastic top sheet or corrugated top cap is applied on the top of the load to lock them in place.

Wrap parameters: The variable settings on a stretch wrapping system that can be adjusted to meet the load retention requirements of the product to be wrapped. These parameters typically include settings like: Top wraps, bottom wraps, film carriage vertical speed, turntable (or arm rotation) speed, and film force.

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